This application is related to subject matter disclosed in:
A1) U.S. provisional App. No. 60/257,218 entitled “Waveguides and resonators for integrated optical devices and methods of fabrication and use thereof” filed Dec. 21, 2000 in the name of Oskar J. Painter;
A2) U.S. non-provisional application Ser. No. 09/788,303 entitled “Cylindrical processing of optical media” filed Feb. 16, 2001 in the names of Peter C. Sercel, Kerry J. Vahala, David W. Vernooy, and Guido Hunziker;
A3) U.S. non-provisional application Ser. No. 09/788,331 entitled “Fiber-ring optical resonators” filed Feb. 16, 2001 in the names of Peter C. Sercel, Kerry J. Vahala, David W. Vernooy, Guido Hunziker, and Robert B. Lee;
A4) U.S. non-provisional application Ser. No. 09/788,300 entitled “Resonant optical filters” filed Feb. 16, 2001 in the names of Kerry J. Vahala, Peter C. Sercel, David W. Vernooy, Oskar J. Painter, and Guido Hunziker;
A5) U.S. non-provisional application Ser. No. 09/788,301 entitled “Resonant optical power control device assemblies” filed Feb. 16, 2001 in the names of Peter C. Sercel, Kerry J. Vahala, David W. Vernooy, Guido Hunziker, Robert B. Lee, and Oskar J. Painter;
A6) U.S. provisional App. No. 60/301,519 entitled “Waveguide-fiber Mach-Zender interferometer and methods of fabrication and use thereof” filed Jun. 27, 2001 in the names of Oskar J. Painter, David W. Vernooy, and Kerry J. Vahala;
A7) U.S. provisional App. No. 60/322,272 entitled “Fiber-optic-taper probe for characterizing transversely-optically-coupled waveguides and resonators” filed Sep. 13, 2001 in the name of David W. Vernooy, said provisional application being hereby incorporated by reference as if fully set forth herein;
A8) U.S. provisional App. No. 60/335,656 entitled “Polarization-engineered transverse optical coupling apparatus and methods” filed Oct. 30, 2001 in the names of Kerry J. Vahala, Peter C. Sercel, Oskar J. Painter, David W. Vernooy, and David S. Alavi;
A9) U.S. non-provisional application Ser. No. 10/037,146 entitled “Resonant optical modulators” filed Dec. 21, 2001 in the names of Oskar J. Painter, Peter C. Sercel, Kerry J. Vahala, David W. Vernooy, and Guido Hunziker;
A10) U.S. non-provisional application Ser No. 10/037,966 entitled “Multi-layer dispersion-engineered waveguides and resonators” filed Dec. 21, 2001 in the names of Oskar J. Painter, David W. Vernooy, and Kerry J. Vahala;
A11) U.S. provisional App. No. 60/334,705 entitled “Integrated end-coupled transverse-optical-coupling apparatus and methods” filed Oct. 30, 2001 in the names of Henry A. Blauvelt, Kerry J. Vahala, Peter C. Sercel, Oskar J. Painter, and Guido Hunziker;
A12) U.S. provisional App. No. 60/333,236 entitled “Alignment apparatus and methods for transverse optical coupling” filed Nov. 23, 2001 in the names of Charles I. Grosjean, Guido Hunziker, Paul M. Bridger, and Oskar J. Painter;
A13) U.S. provisional App. No. 60/360,261 entitled “Alignment-insensitive optical junction apparatus and methods employing adiabatic optical power transfer” filed Feb. 27, 2002 in the names of Henry A. Blauvelt, Kerry J. Vahala, David W. Vernooy, and Joel S. Paslaski;
A14) U.S. non-provisional application Ser. No. 10/187,030 entitled “Optical junction apparatus and methods employing optical power transverse-transfer” filed Jun. 28, 2002 in the names of Henry A. Blauvelt, Kerry J. Vahala, David W. Vernooy, and Joel S. Paslaski;
P1) “Carbon Dioxide laser fabrication of fused-fiber couplers and tapers”, T. Dimmick, G. Kakarantzas, T. Birks and P. St. J. Russell, Applied Optics 38, 6845 (1999); and                U.S. Pat. No. 5,101,453 entitled “Fiber Optic Wafer Probe” issued Mar. 31, 1992 in the name of assignee Cascade Microtech.        
With the development of techniques for efficient transverse-transfer of optical power between a fiber-optic taper and an optical waveguide fabricated on a substrate (including, for example: semiconductor-based optical DBR waveguides or resonators, as in applications A1, A6, A8, and A10 cited above; external-transfer waveguides integrated with optical devices, as in applications A11, A13, and A14 cited above; methods and apparatus disclosed herein may be suitable for other transversely-optically-coupled optical components as well), the need arises for wafer-scale fiber-taper-based optical testing probes. Free space-to-chip or cleaved-fiber-to-chip (e.g. “butt” coupling or “end-fire” coupling) testing techniques may be practically dismissed as too cumbersome, too inefficient, and generally unsuitable for large-scale (i.e., many optical devices per substrate) testing rigs.
Since the optical components to be tested are intended to be transversely-optically-coupled to one or more other optical components (often including a fiber-optic taper) of a packaged device, it is important that pre-characterization and/or pre-qualification of the optical structures be performed under similar conditions, e.g., transversely optically coupled to a fiber-optic taper. Transverse optical coupling may also be referred to as evanescent optical coupling or optical power transverse-transfer, and is described in detail in patent applications A1 through A14. A difficulty with straight-taper-based testing is essentially determined by geometry. A typical fiber-optic taper diameter decreases from about 125 μm to about 2.5 μm in a length of about 30 mm. A typical ridge waveguide may extend only about 3-4 μm above the substrate. Efficient coupling typically requires contact between the taper and ridge waveguide. Assuming the tapering segment of the fiber has a diameter decreasing roughly linearly with distance, then the overall substrate width should be less than about 500 μm so that the larger diameters of the tapering segment of the fiber do not interfere with the placement of the thinnest center taper segment on the ridge waveguide.
In practice it has proven difficult, on samples wider than about 500 μm, to establish transverse optical coupling between the thinnest segment of the taper and a ridge or planar waveguide on the substrate, while avoiding contact between larger-diameter segments of the fiber and the substrate. As a result, reliable and/or repeatable coupling of the taper and the waveguide for testing is nearly impossible using a straight taper and an uncleaved substrate. Furthermore, even in cases where the geometry is marginally favorable, incidental contact of the larger portions of the taper and the substrate can cause additional insertion loss for the device being tested. From both reliability and performance standpoints, it is desirable to avoid any unnecessary contact of the tapered section of the optical fiber with the substrate.
This imposes extreme constraints on preparation techniques for a multiple-waveguide sample before it can be optically characterized. For example, at the very least scribe-and-break techniques are necessary prior to mounting the substrate sample in order to test the optical quality of the waveguides fabricated thereon. In extreme cases, lapping and cleaving may be required in order to ensure accurate sample preparation (e.g., flat end facets) and maximization of chip yield. While both of these techniques tend to introduce particulates onto the wafer substrate at a very late stage in the processing, this is not the most important problem associated with the need to subdivide the wafer for optical testing. It is of utmost importance to pre-test and pre-qualify the optical quality of the guides in a manner as rapid, accurate, and preferably automated as practicable before beginning the labor-intensive process of subdividing the chip substrate.
As an alternative solution to the problem of contact between the optical fiber and the substrate, one or more “trenches” or other similarly depressed spatial regions may be provided on the substrate near the optical components to be tested. Such depressed regions may reduce or eliminate contact between the tapering portions of the optical fiber and the substrate. Such regions may be provided near each of multiple optical components fabricated on a common substrate. Such a solution entails the added effort and expense of the additional fabrication steps to produce the depressed regions on the substrate, and the necessarily reduced ultimate yield of qualified optical components.
It is highly desirable, perhaps essential, that an optical probe not interfere with or limit testing of other aspects of a waveguide device on a substrate. For example, the optical guide may form part of an active optical device (e.g., modulator, laser, detector, etc) for which DC (parametric) electrical, high speed (RF, microwave) electrical, thermal, and/or mechanical testing may be required, concurrently with optical testing and/or at a later stage. Furthermore, an optical probe should be mechanically compact and stable, preferably should not interfere with cameras or microscopes used in automated image recognition, and preferably may be integrated with standard electro-mechanical positioning stages. It is highly desirable to use the considerable commercial infrastructure already in place for automated testing. Hence, compatibility of the optical probe with such machinery is extremely important.
It should be noted that a commercial vendor (Cascade Microtech) supplies probing heads for standard optical probe stations under the name “fiber optic probes” (U.S. Pat. No. 5,101,453). This probe is typically used for on-wafer testing of VCSEL devices and, with some modifications, stripe lasers and laser bars. These probes are essentially cleaved fibers mounted on a standard probe station head and would perform essentially as “butt-coupled” or “end-fire” fiber probes for the present application. Butt coupling is not suitable for testing transversely-optically-coupled components according to the present invention, and the prior probes are not suitable for transverse optical coupling.
It is therefore desirable to provide apparatus and methods for optically characterizing transverse-coupled waveguides, resonators, and/or other optical components, particularly transverse-coupled components on substantially planar substrates (including substrates greater than a millimeter across). It is desirable to provide such apparatus and methods that may be implemented for characterizing many optical components on a common wafer-scale substrate, the components being adapted for transverse optical coupling.